JP 2000-121404 A discloses an unit for measuring a flow rate comprising a gas flow path; ultrasonic sensors for transmitting and receiving which are installed on the gas flow path; a controlling measurement circuit for controlling the transmitting and receiving of the ultrasonic sensors and measuring an ultrasonic propagation time, and further for transmitting and receiving measured data through data bus lines; and a non-volatile memory for memorizing a part of control contents in the controlling measurement circuit, wherein measurement of a true flow quantity is performed at every unit part of the constituents, calculation of the flow quantity is performed by performing transmitting and receiving data by means of a personal computer for processing data which is provided separately, specific amendment data of every unit part are written in the non-volatile memory from comparison to the true flow quantity to maintain the specific amendment data. As shown in JP 2000-121404 A, it is disclosed that the principle of the ultrasonic-style flow quantity detection, the propagation time of an ultrasonic wave between two points in the gas flow path, is a function including the gas flow velocity, such that the gas flow velocity can be calculated backward if the propagation time is measured, and the flow quantity can be calculated if the flow velocity and the passage sectional area are found and applied.
JP 2010-14690 A discloses an ultrasonic flowmeter that, in order to achieve flow measurement with a high degree of accuracy and superior in responsiveness, at a preliminary measurement for measuring a propagation time of an ultrasonic wave previously, the number of clock pulses measured from a time of ultrasonic wave transmitting to a time of ultrasonic wave receiving is memorized in a forward direction and in a reverse direction, and at the time of a main measurement after the preliminary measurement, counting the number of the clock pulse are starts from the time of ultrasonic wave transmitting, integration starts at a time of outputting clock pulses before two pulses of the number of clock pulses memorized in the forward direction and the reverse direction, and integral calculus voltage is measured at a time of reaching the received wave and a minute time is calculated, so that the propagation time is measured due to the clock pulse time and the minute time.
JP 2001-7539 A discloses an ultrasonic flowmeter that comprises a plurality of ultrasonic vibrators which is disposed on a measurement pipe with a specific distance along fluid flow and an operation circuit for calculating the velocity of a flowing fluid based on a time lag between a propagation time until the downstream ultrasonic vibrator receives the ultrasonic wave transmitted from the upstream ultrasonic vibrator and a propagation time until the upstream ultrasonic vibrator receives the ultrasonic wave transmitted from the downstream ultrasonic vibrator, wherein the ultrasonic vibrator transmits an ultrasonic wave with a first frequency and an ultrasonic wave with a second frequency determined previously corresponding to the measurement pipe alternately, and the operation circuit calculates a first time lag of the propagation time in the ultrasonic wave with the first frequency as a time lag when the flow velocity is zero, so that the flow velocity is calculated based on the first time lag and the second time lag of the propagation time in the ultrasonic wave with the second frequency.